Rollup merge of #69668 - ssomers:btreemap_even_more_comments, r=Mark-Simulacrum
Rollup merge of #69668 - ssomers:btreemap_even_more_comments, r=Mark-Simulacrum

More documentation and simplification of BTreeMap's internals

Salvage the documentation and simplification from #67980, without changing the type locked down by debuginfo.

r? @rkruppe

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust